1. Field of the Invention
The invention relates generally to a process for the manufacture of moldings, and more particularly to a process for the manufacture of catalyst moldings.
2. Description of the Prior Art
Such catalyst moldings are used for the reduction of nitrogen oxides in exhaust gases produced by combustion equipment. According to the prior art, the quantity of nitrogen oxides can be reduced by means of reducing agents such as CO, NH.sub.3, CH.sub.4, H.sub.2, etc. The prior art also includes catalyst materials such as TiO.sub.2, WO.sub.3, V.sub.2 O.sub.5, MoO.sub.3, Cr.sub.2 O.sub.3, Fe.sub.2 O.sub.3 or mixtures of these materials, as well as catalytic additives such as WO.sub.3, V.sub.2 O.sub.5, MoO.sub.2 and Cr.sub.2 O.sub.3 as active substances. Additives in highly-pure form for use with carrier substances are expensive, and, because of their high activity, are sensitive to aging. They are also difficult to dispose of. The recovery of the valuable active substances is naturally desirable, but it entails a significant expense for equipment, which therefore has an adverse effect on the economy of the process.
Attempts have already been made to manufacture Fe.sub.2 O.sub.3 catalyst material using sulphuric acid or phosphoric acid as binders. It has thereby been shown that only the sulphuric acid binding leads to a high activity, but it also produces an excessive sensitivity to water, which causes the moldings to absorb water and become soft. On the other hand, the phosphoric acid bonding reduces activity, but produces hard moldings which are not sensitive to water. Mixtures of phosphoric acid and sulphuric acid lead to a catalyst action which is not always satisfactory. Even the saturation of catalyst moldings containing a phosphoric acid binder with sulphuric acid, and vice-versa, results in only moderate improvements.
In the manufacture of catalyst moldings, it is frequently necessary to produce thin webs for plates, honeycombs, tubes and other configurations, which means that sufficient strength must be assured. This normally requires relatively high pressures with a homogeneous pressure distribution. Up to now, it has been impossible to achieve these caracteristics without a major expenditure for equipment. Therefore, molding manufacturing processes of the prior art suffer from the disadvantage that the molding must be removed from the mold very carefully and must be subjected to a long tempering process, to achieve a strong and durable shape. Attempts have also been made to use intermittent molding techniques or calendering. But here again, it turns out that the molding is either impossible to work with, or can only be worked with with great difficulty. Specifically, it is difficult to remove the molding from the mold, and undesirably long tempering times are necessary.
Catalysts are known, specifically, for example, catalysts containing Fe.sub.2 O.sub.3, Cr.sub.2 O.sub.3 or V.sub.2 O.sub.5. In terms of their action, however, these catalyst leave something to be desired.
Some examples of catalytic technology are found in U.S. Pat. No. 4,259,312, entitled "Process And Apparatus For Catalytically Reacting A Reducing Gas And Water Vapor"; U.S. Pat. No. 4,235,604, entitled "Method For Processing Coke Oven Gas"; and U.S. Pat. No. 4,049,777, entitled "Method Of Waste Gas Treatment", all of which are assigned the same assignee as the instant application.
Further examples of catalytic technology are found in U.S. Pat. No. 4,448,895, entitled "Process For Preparation Of Catalyst For Cleaning Exhaust Gases And Catalyst Prepared For The Process", and U.S. Pat. No. 4,503,162, entitled "Catalyst Systems For The Conversion Of Gases".
Examples of reactivation of catalysts useful in nitrogen oxide removal are described in U.S. Pat. No. 4,044,102, entitled "Method For Treating Exhaust Gases", and U.S. Pat. No. 4,043,939, entitled "Reactivation Of Catalysts Useful For Nitrogen Oxide Removal." All of the above-cited patents are incorporated herein by reference as if the texts thereof were fully set forth herein.